The invention relates to a check valve assembly and a valve assembly containing a check valve assembly.
In mobile hydraulics, such valve assemblies are i.a. used for controlling double-action pressure cylinders, such as employed, e.g., as front power lifts or rear power lifts of farming tractors. By means of these power lifters it is possible to operate various peripheral tools such as, e.g., cutter bars, balers, ploughs, cultivators, rollers etc.
Under certain operating conditions, a situation may arise in which a load must be sustained in the lifted condition, where it is desired for this condition to be maintained even when the hydraulic supply is deactivated. Thus, for example, a cutter bar must be immobilised in the lifted condition while the farming tractor travels to the meadow to be mown or while it is parked in the farmyard.
Customarily the power lifters are controlled by means of proportional valves possessing a Zero position wherein the work conduits communicating with the power lifters and a pump conduit are shut off. Such proportional valves are, however, generally designed as sliding valves, so that the hydraulic fluid cannot be shut off leak-free in the work conduits. Consequently, piloted or releasable check valves are provided in the work conduits, whereby the hydraulic fluid may be confined leak-free between the user and the check valve. By releasing the check valve, a return flow of the hydraulic fluid to the proportional valve is enabled.
DE 27 35 559 C2 discloses a valve assembly wherein a main poppet may be controlled open for release with the aid of a topping piston. The main piston is designed with a pilot hole which is closed by a closing member. The latter may be taken into its opened position by controlling the topping piston through the intermediary of a tappet, so that the spring side of the main poppet is de-pressurised and the main poppet is controlled open by the load pressure applied to its differential surfaces. At the same time, the topping piston controls open a tank port, so that the hydraulic fluid may flow from the user past the raised main poppet to the tank T without requiring a return connection to a directional control valve arranged upstream of the check valve assembly. By the known combination of directional control valve and check valve assembly, merely a single-action lifting cylinder may be connected, the cylinder chamber of which may optionally be connected through the control valve to a pump or through the topping piston to the tank.
With the closing member in the raised condition, the main poppet is controlled open instantaneously due to its area difference, so that the hydraulic fluid is de-pressurised toward the tank comparatively rapidly, without a possibility of influencing the velocity of return flow.
In DE-44 36 548 A1 a valve assembly for controlling double-action lifting cylinders is disclosed, wherein a check valve assembly is arranged in every work conduit. The valve assembly in turn contains a main poppet designed with an area difference (i.e., the valve seat diameter is smaller than the effective external diameter of the main poppet), which may be controlled open by means of a topping piston. Due to this valve assembly it is possible to control a double action user, however, as the main poppet is again designed with an area difference, the main poppet immediately following rising from its valve seat is instantaneously set to its open position, so that a maximum cross section of flow is provided at once. In this known valve assembly, too, it is not possible to influence the return flow.